1. Field of the Invention
The present invention relates generally to downhole tandem drilling motors of the progressive cavity type and, particularly, to a flexible coupling for connecting the upper and lower rotors and stators of the downhole tandem motor.
2. Description of Related Art
Downhole drilling motors are used in the drilling of oil and gas wells. In the usual mode of operation, the power output shaft of the motor is connected to a drilling tool such as a drill bit which then rotates with respect to the housing of the motor. The housing, in turn, is connected to a conventional drill string composed of drill collars and sections of drill pipe. This drill string extends to the surface where it is connected to a kelly mounted to the rotary table of a drilling rig. Drilling fluid is pumped down through the drill string to the bottom of the hole and back up the annulus between the drill string and the wall of the borehole. The drilling fluid cools the drilling tool and removes the cuttings resulting from the drilling operation. The drilling fluid also provides the hydraulic power to operate the motor.
Downhole drilling motors of the progressive cavity type are well known in the art. Such a motor includes a helicoidal rotor and a complimentary helicoidal stator forming a power section. The housing of the stator typically includes a conduit made of steel and a helically grooved elastic lining bonded to the inner wall of the conduit forming rubber lobes. The rotor is also typically made of steel with chrome plating and is disposed inside the stator. The rotor likewise is provided with helical grooves on its outer surface, the number of rotor grooves typically being one less than the number of stator grooves. The rotor includes a central passageway therethrough for the passage of fluid with the lower end of the rotor bring connected to an output shaft. The helical grooves of the stator and rotor form cavities of slightly variable volume for the passage of fluid therethrough. The working fluid is forced to flow between the stator and rotor so as to fill the cavities and act on the helical grooves of the rotor causing the rotor to rotate within the stator as the cavities move down the length of contact between the rotor and stator.
As the helical rotor rotates in an eccentric fashion, the rotor gyrates and rotates within the stator at a distance from the stator center line in the reverse direction relative to its rotation. A universal connection is used to connect the gyrating rotor to the non-gyrating output shaft and translate this gyrating motion into true rotational motion. A typical universal connection includes a pair of universal sections which connect a straight rod to the rotor and to the output shaft. The universal sections are designed to take only torsional load and have a ball and race assembly to take thrust load. Rubber boots may be clamped over the universal sections to keep out drilling fluid.
Since the drilling fluid for drilling the borehole serves both the function of cooling the drill bit and removing the cuttings as well as operating the downhole motor, it is necessary to have proper fluid flow to both the bit and through the motor. For this purpose, a nozzle is typically placed at the top of the rotor to regulate the flow and apportion the flow between the central passageway in the rotor and the flow path between the rotor and stator. There is also a nozzle in the lower end of the rotor to allow the flow through the central passageway of the rotor to combine with the flow between the rotor and stator prior to passing through the nozzles of the drill bit. Where it is desirable to increase the flow through the drill bit to allow greater cleaning of the bit due to the accumulation of cuttings, the size of the nozzle at the top of the rotor is changed to adjust that part of the flow through the power section so that the preferred flow rate can be maintained through the power section of the motor.
The speed of the motor is determined by the ratio of the gallons of fluid per minute (GPM) which flow through the motor to the revolutions per minute (RPM) of the drill bit. A predetermined fluid volumetric velocity (GPM) and pressure at the bit nozzles is required so that the cuttings may be moved through the annulus to the surface. Further, the torque of the motor is proportional to the pressure drop across the stator. As the fluid passes through each stage of the power section, the fluid pressure is reduced. This pressure drop is required to add resistance to the flow to convert into torque to rotate the rotor. These considerations influence the minimum pressure drop which can be tolerated and still obtain the necessary fluid velocities and pressures at the bit nozzles.
The rubber of the stator frequently fails due to the eccentric motion of the rotor and the magnitude of the pressure drop across the motor if the motor delivers a substantial torque or if there is too much flow through the power section. The resultant hysteresis in the rubber deleteriously affects the properties of the rubber. Further, if the rotor spins too fast, the centrifugal force of the rotor causes it to bow outwardly and tear the rubber on the stator. The result is a loss of portions of the rubber which break away from the body of the stator, called "chunking," and may strip the rubber away from the metal housing due to bond failure.
The length of the complimentary surface areas between the upper rotor-stator power section and lower rotor-stator power section has a direct effect on the power output of the motor. Thus, by lengthening the power section, the torque of the motor is increased. See for example, U.S. Pat. No. 5,090,497. Although lengthening the power section achieves more power, there are manufacturing and assembly problems in making the power section longer. The molding of the rubber to produce a successful bond to the stator housing and the necessary helical configuration at its surface become more difficult as the diameter or length of the stator increase. Another difficulty is holding the stator rubber in a long tube and keeping the rubber centralized. Another problem is the length and flexibility of the stator in smaller motors. Also, as flexibility is increased, the manufacture of the elongated power section parts becomes more expensive and less reliable.
One means of obtaining more power and avoiding a lengthy rotor and stator is the use of a tandem motor which includes an upper power section and a lower power section. U.S. Pat. Nos. 3,982,858; 4,585,401; and 4,711,006 disclose tandem motors. In a tandem motor, the upper and lower rotors and stators are connected. The rotors are connected together by constant velocity joints that allow free radial movement and/or offset and/or bending between the two attached rotors. Constant velocity joints are similar to the universal joints previously described where there is no side load. The stators are connected by a spacer housing.
In the prior art tandem motors, the fluid must bypass and flow around the constant velocity joint. There is no room in the interior diameter of the constant velocity joint to have a flow passage. Flowing the fluid around the joint does not work well in a high flow motor because it is necessary to have a nozzle at the upper end and ports at the lower end of the upper rotor and another nozzle at the upper end of the lower rotor to allow the flow to bypass and flow around the joint. The flow from the surface passes out the ports in the lower end of the upper rotor, through the annulus between the connecting rod and the spacer housing, and then flows through the nozzle in the upper end of the lower rotor.
A solid connecting rod may be used between the upper and lower power sections in place of the constant velocity joint. However, the nozzle of the upper rotor and the nozzle back of the lower rotor must be properly sized. Problems can occur if the nozzles are not sized properly. Each nozzle is sized for a given flow rate for the working pressure of that power section. If the nozzle in the upper rotor is sized to have a lesser flow rate than the nozzle in the lower rotor, it will cause an increased flow rate through the upper power section, that will cause an increased pressure drop and flow rate through the upper power section that can be detrimental as described earlier. Generally, the upper nozzle must be slightly smaller than the lower nozzle because oftentimes the upper power section is shorter than the lower power section. For example, in medium speed motors, the upper power section is typically about one half the length of the lower power section. When the upper power section is half the length of the lower power section, the pressure drop across the upper power section is about half that of the lower power section.
Tandem downhole drilling motors are often used to drill deviated wells or horizontal wells. Such a motor is often referred to in the industry as a steerable motor. A steerable motor and bottom hole assembly is described in U.S. Pat. No. Re. 33,751 dated Nov. 26, 1991 to Geczy. A steerable drilling motor typically includes a bent housing located below the lower power section and the motor is steered in the desired direction by alternately rotating the motor housings to achieve the desired hole curvature.
Whenever the motor is rotated in a curved section of the borehole, or whenever it is reoriented in a straight section of the borehole, there is an interference between the motor housing and the borehole causing the motor to flex or bend due to the side loads caused by the borehole. The bend of the motor housing may be so large that there is difficulty getting the motor in and out of the borehole. Thus, the stator housing and rotor must bend along with their working surface profiles. The stator housing and rotor are made of steel or some other stiff material and thus excessive loads are placed on the rotor and stator at the ends and near the middle of the power section.
With the more recent downhole motors being designed with increased torque capacity, more length and more stages of the power section are added to the motor. This added length subjects the motor housings to additional flexing and is more difficult to steer. The flexing causes additional side loads to be applied between the rotor and stator rubber. Also, as a result of the higher penetration rates, cuttings from the bit accumulate faster than previously and require higher flow rates through the motor and up the annulus to float the cuttings to the surface and out of the hole quickly enough to prevent a build up of cuttings that may cause the drill string and motor to get stuck in the hole and/or slow down the rate of penetration. The power section is limited to a maximum flow rate before fluid velocities become large enough to destroy the stator rubber and wash out the power section.
One prior art method of providing the necessary flexibility between the power sections of a tandem motor is the use of a titanium rod as the connecting rod between the upper and lower rotors. Each end of the titanium rod is shrink fitted into the mating ends of the upper and lower rotors. The titanium rod flexes upon the application of a side load to the tandem motor and thus allows a limited side load providing flexibility in the steerable tandem motor. The titanium rod, however, is a solid rod and still requires that the fluid bypass and flow around the titanium rod between the upper and lower rotors. It is very costly to put a central passageway through a titanium connecting rod. Further, there is insufficient room within the spacer housing to provide an adequate shrink fit between the titanium connecting rod and the upper and lower rotors. The shrink fits on the end of the titanium connecting rod tend to come loose and not transmit the required torque between the upper and lower motors. Occasionally, the titanium connecting rod slips or breaks.
One of the problems with using the titanium connecting rod is that the tandem motor must be disassembled to change out a worn out rotor power section. The shrink fit connections between the ends of the titanium rod and the upper and lower rotors become loose from disassembly and assembly.
The present invention overcomes the deficiencies of the prior art.